THE STRUCTURAL CLASSIFICATION 587 



basaltic cones. One case was seen where asphaltic oil was flowing down 

 the side of a cone from a breccia included in the basalt 50 to 60 feet above 

 the surrounding plain. It is supposed that this oil entered the basalt 

 through fissures, which extend into the plug from the oil sand, and that 

 its passage through the basalt was caused by the great pressure under 

 which it existed. 



Volcanic necks of basalt are scattered at wide intervals throughout the 

 Gulf Coastal Plain of Mexico. The greatest center of volcanic activity 

 was the Otontepec and Tantima Mountains, several thousand feet in 

 height, in the State of Vera Cruz. The volcanic activity seems to have 

 become less at increasing distances from these mountains and decreased 

 almost entirely before reaching the Eio Grande far to the north. The 

 majority of the plugs are only a few hundred feet in height and some of 

 them less than 100 feet, and it is probable that many exist which never 

 reached the surface. The geological relations of the basalt renders it 

 undoubtedly of more recent origin than the Coastal Plain sediments ; and 

 although frequently no disturbance can be discovered surrounding the 

 plugs, there is no doubt that such disturbances do exist and that Subclass 

 TV{d) is a necessity. 



No igneous rock has been definitely proven in saline domes; but Cap- 

 tain Lucas thought he had igneous rock beneath the salt in a 3,300-foot 

 well at Belle Isle, Louisiana.^^ That the volcanic neck type is presumably 

 more common than is yet known is evinced by the fact that on the south- 

 ern edge of the Transylvanian basin in Hungary, where saline domes are 

 the prevailing type, arranged in linear series similar to those of Lou- 

 isiana, is one prominent instance where a plug of basalt rises above the 

 plain instead of the saline dome which is due at the particular point. 



Subclass IV(e) — Perforated saline domes. — In Transylvania and Rou- 

 mania the saline dome type of structure has frequently reached an exag- 

 gerated phase, owing to the fact that the dome-shaped salt masses have 

 reached the surface of the earth, and that the surrounding strata have 

 been compressed outward to such an extent that they stand vertical, or 

 even are overturned in a narrow belt surrounding the dome. In Rou- 

 mania large oil fields are found in such structures, which were originally 

 described by Professor Mrazek as "diapir structure" or perforated domes. 

 While this type may exist in America, it is not known to the writer as 

 oil-bearing. In New Brunswick certain gypsum deposits appear to be of 

 similar structure and probable origin, but are not supposed to contain oil. 



Perforated domes may be considered as a class intermediate between 



A. F. Lucas : Trans. Am. Inst. MIn. Engrs., vol. 48, p. 693. 

 XLV — Bull. Geol. Soc. Am., Voi<, 88, X916 



